1. Field of the Invention
The present invention concerns a conveyor device used for electrostatic coating by applying a high voltage on a work to be coated instead of applying the high voltage on a coating machine.
2. Description of the Prior Art
Electrostatic coating of a work such as a car body has been conducted so far by applying a high voltage of from -80 V to 120 kV on an electrostatic coating machine that jets out paint while to the earth grounding a work placed on a conveyor truck.
In this case, electrostatic coating is generally applied by using an insulative paint. Movements for environmental protection have become popular world wide in recent years and it has been keenly advocated to regulate the use of organic solvents such as hydrocarbons that yield substances causing public pollution and, in compliance therewith, it has been recommended also in the industrial field of coating to cease the use of insulative paints that consume a great amount of deleterious organic solvents such as thinner and transfer to electrostatic coating using aqueous paints causing no public pollution.
However, when an electroconductive paint such as an aqueous paint is sprayed by an electrostatic coating machine applied with a high voltage, it is necessary to provide an adequate means for the insulation of a paint supply system so as not to leak a high voltage applied to the coating machine (refer to Japanese Patent Laid Open Sho 55-114366 and Sho 56-141869). This results in considerable troubles to a coating facility of car bodies for conducting multi-color coating while changing colors of paints among several tens of colors, in that each of the paint supply pipelines and paint reservoirs has to be insulated.
In addition, it is also necessary to provide a safety means for the paint supply system applied with the high voltage, such as by surrounding the periphery of the paint supply pipe lines or paint reservoirs with protection chambers, but this results in a problem of remarkably increasing the cost for insulation and safety and enlarging the scale of the entire facility.
In view of the above, the applicant has already proposed a conveyor device capable of applying a high voltage on a work so that electrostatic coating can be conducted using an electroconductive paint without providing insulation means for the paint supply system (Japanese Utility Model Laid-Open Hei 3-105948 and Hei 4-16796).
FIG. 10 illustrates such a proposed conveyor device, in which a work W is placed on a truck 71 running on a rail 70, and an electrode attachment 72 for applying a high voltage to the work is attached in an insulated state by means of insulation posts 73, such as ones made of porcelain, for keeping a necessary insulation distance. A current collector rail 74 is disposed along a transporting direction of the truck 71 in a high voltage application zone for conducting electrostatic coating under application of a high voltage to the work, and a contact 75 connected to the attachment 72 is in sliding contact with the collector rail 74.
When the truck 71 comes to the high voltage coating zone, the contact 75 is brought into sliding contact with the collector rail 74 to apply the high voltage on the work W, so that electrostatic coating can be conducted without providing insulation means to the electrostatic coating machine, the paint supply system, the floor conveyor or the like.
However, since the high voltage is always supplied to the collector rail 75, sparks are generated on every instance of contact and detachment between the contact 74 of the truck 71 and the collector rail 75, as well as during sliding contact between the contact 74 and the collector rail 75 caused by attachment and detachment between them.
If sparks are generated, surfaces of the contact 74 and the collector rail 75 are melted and made uneven by the heat, which tends to cause more sparks. This enforces frequent exchange of the contact 74 and the collector rail 75 to bring about a problem of increasing the maintenance cost.
In addition, since there is a great potential difference between the collector rail 75 and the coating machine, an electrostatic field is formed between them to cause the paint liable to deposit on the collector rail 75, so that the rail has to be cleaned frequently and this brings about considerable troubles in the maintenance.
For overcoming such problems, it has been proposed to dispose the collector rail in a groove filled with an insulating oil for covering the surface of the collector rail with the insulating oil to thereby prevent the generation of sparks or prevent the formation of the electrostatic field (refer to Japanese Patent Laid-Open Hei 4-61944).
However, the insulating oil, if it is used in the coating zone, would splash and deposit on the work, which repels the paint and brings about a problem of defective coating.
Further, it has also been proposed to once transfer a work to an insulated conveyor disposed in a high voltage application zone and then apply a high voltage (refer to Japanese Patent Laid-Open Hei 3-224651 and Hei-44-225857).
However, this complicates and enlarges the scale of the facility and requires provision of voltage increasing and decreasing zones before and after transfer steps so that sparks may not be generated upon transfer to the insulated conveyor, which results in a problem of troublesome voltage control and requiring a large space.